This proposal requests funds to purchase two shared instruments: a Hewlett Packard Gas Chromatograph/Mass Spectrometer/Data System and a Hewlett Packard Gas Chromatograph. These instruments will be used by a large group of clinical investigators affiliated with the Massachusetts Institute of Technology Clinical Research Center (MIT CRC). These two instruments, in addition to a gas isotope ratio mass spectrometer and an amino acid analyzer system which are being requested in a separate proposal, will form a major shared-instrument facility at the MIT CRC. The research of the major user group encompasses three areas, all of which have been highly productive: 1) Stable isotope tracer probes of amino acid and substrate metabolism under conditions of health, nutritional inadequacy, aging, and severe metabolic trauma and stress, 2) brain function and neurotransmitter synthesis in response to dietary factors such as amino acids, carbohydrate, and food additives (aspartame), and 3) brain function and behavior in several diseases, especially Alzheimer's disease. The major application of stable isotope methodology has been in the first area. At present GC/MS analytic needs for amino acid and substrate metabolism are met by a complex network of collaboration with mass spectrometry facilities at other institutions. Further expansion of this research program is limited, not by research funding but by heavy competition for existing analytic capabilities. Research in the other two areas would be greatly enhanced by applying new stable isotope tracer techniques and mass spectrometry analytic methods in ways outlined in the proposal, but unfortunately, limited GC/MS analytic resources make this impossible unless new resources are acquired. Therefore, these instruments have been requested to relieve a critical limitation in mass spectrometry analytic capability, as well as to facilitate the development of new applications of mass spectrometry to other exciting areas of human brain function and metabolism. The location of the shared facility in proximity to the MIT CRC will promote training of young clinical investigators in this powerful new technology.